It is known that structures having two or more portions that are affixed together can separate during the service life of the structure. For example, in a laminate structure, separation by delamination of the strata within the laminate can occur. There are many causes of separation, including the bond joining portions of the structure weakening over time, impurities in the material, and excessive stress experienced when the structure is under load.
Known methods for non-destructive testing (NDT) of a structure for separation include frequency-response analysis, in which the resonant modal frequencies of the structure in response to an excitation vibration are analyzed.
Another NDT method involves laser shearography, in which a surface is measured using a laser with and without an applied load in order to measure the geometry difference.
Another alternative NDT method for testing for delamination involves delamination thermography, in which heat transfer through a structure is used to determine discontinuities or air gaps within the structure.
These methods are relatively effective for detecting separation. However, it is recognized that difficulties arise when attempting to employ these methods while the structure is in situ, or in operation.